This invention relates to producing an aluminum electrolytic capacitor having a stable oxide film, and more particularly to producing such a capacitor having an aluminum oxide dielectric that resists interaction with water.
It is known in the electrolytic capacitor art that the formation of dielectric oxide on an aluminum electrode is facilitated by first producing a hydrated oxide film on the electrode and then anodizing the electrode in a formation electrolyte. The formation of the hydrated oxide serves to reduce the electrical energy requirements for the subsequent anodization; while some hydrate is consumed during anodization, some is left over and serves to increase the equivalent series resistance and decrease the capacitance available from the anodized electrode. The capacitance decrease is caused by hydrous oxide plugging of the fine etch structure of high voltage foils. One means for coping with both the advantages and disadvantages of the hydrated oxide is disclosed in U.S. Pat. No. 3,733,291 issued May 15, 1973, wherein the formation process includes the stripping away of any hydrated oxide that remains after anodization.
It is known to depolarize after anodization in the presence of hydrated oxide to deal with an instability evidenced by a sudden loss of field strength after apparently complete film formation. It has been postulated that this instability is caused by gas bubbles trapped in the hydrous oxide layer. Others question this on the basis that a random occlusion of gas bubbles would not account for certain regularities in instability behavior, but do agree there are some sort of voids in the formed film. However, there is general agreement that the unstable state is related to the presence of the hydrous oxide.
Whatever the cause, it is known to remedy the situation by depolarizing techniques-- heating, immersion in hot water, secondary anodization, mechanical flexing, pulsed currents, or current reversal-- in short, methods which tend to crack the barrier oxide layer slightly.
If the hot water immersion method is used, hydrate in excess of that present initially will be formed if the anodization is carried out using the standard boric acid and/or borate electrolytes. These electrolytes enjoy wide commercial utilization in the formation of dielectric oxide films on aluminum electrodes for use in electrolytic capacitors because of their efficiency and low cost. However, W. J. Bernard and J. J. Randall, Jr., have shown in J. Electrochem. Soc. 108, 822, (1961) that the resulting oxide film thus formed is attacked by water to form a non-insulating hydrous oxide. The degradation of the oxide film can occur also by the action of water in rinse baths, in the working electrolyte of the capacitor, or even from exposure to air.
While hydrate formation can be inhibited prior to anodization, and this is of particular importance for low-voltage foils, hydrate formation on high-voltage foils is desirable to reduce energy requirements during anodization. In general these inhibition processes of the prior art require more inhibitor than the present invention.